This invention relates to cleaning of screens, gravel packs and formations in producing well having installed production tubing. In particular, this invention relates to methods that provide downhole acoustic cleaning.
In any typical hydrocarbon well, damage to the surrounding formation can impede fluid flow and cause production levels to drop. While many damage mechanisms plague wells, one of the most pervasive problems is particles clogging the formation pores that usually allow hydrocarbon flow. These clogging particles can also obstruct fluid pathways in screens; preslotted, predrilled, or cemented and perforated liners; and gravel packs that may line a well. Clogging particles may even restrict fluid flow in open-hole wells. Drilling mud, drilled solid invasion, or even the porous formation medium itself may be sources for these particles. In particular, in situ fines mobilized during production can lodge themselves in the formation pores, preslotted liners, screens and gravel packs, sealing them to fluid flow. Referred to as the “skin effect,” this damage is often unavoidable and can arise at any stage in the life of a typical hydrocarbon well. The hydrocarbon production industry has thus developed well-stimulation techniques to repair affected wells or at least mitigate skin-effect damage.
The two classic stimulation techniques for formation damage, matrix acidizing and hydraulic fracturing, suffer from limitations that often make them impractical. Both techniques require the operator to pump customized fluids into the well, a process that is expensive, invasive and difficult to control. In matrix acidizing, pumps inject thousands of gallons of acid into the well to dissolve away precipitates, fines, or scale on the inside of tubulars, in the pores of a screen or gravel pack, or inside the formation. Any tool, screen, liner or casing that comes into contact with the acid must be protected from its corrosive effects. A corrosion inhibitor must be used to prevent tubulars from corrosion. Also, the acid must be removed from the well. Often, the well must also be flushed with pre- and post-acid solutions. Aside from the difficulties of determining the proper chemical composition for these fluids and pumping them down the well, the environmental costs of matrix acidizing can render the process undesirable. Screens, preslotted liners and gravel packs may also be flushed with a brine solution to remove solid particles. While this brine treatment is cheap and relatively easy to complete, it offers only a temporary and localized respite from the skin effect. Moreover, frequent flushing can damage the formation and further decrease production. In hydraulic fracturing, a customized fluid is ejected at extremely high pressure against the well bore walls to force the surrounding formation to fracture. The customized gel-based fluid contains a proppant to hold the fractures open to fluid flow. While this procedure is highly effective at overcoming near-borehole skin effects, it requires both specialized equipment and specialized fluids and therefore can be costly. Fracturing can also result in particle deposition in the formation because the gels involved may leave residue in the vicinity of the fractures.
The hydrocarbon production industry developed acoustic stimulation as an alternative to the classic stimulation techniques. In acoustic stimulation used for near-well bore cleaning, high-intensity acoustic waves transfer vibrational energy to the solid particles clogging formation pores. The ensuing vibrations of the solid particles loosen them from the pores. Production-fluid flow out of the formation in producing wells causes the solid particles to migrate out of the pores, clearing the way for greater production-fluid flow. In injection wells, either injection-fluid flow or production-fluid flow can flush the loosened solid particles from the pores. Acoustic stimulation may also be used to clean preslotted and predrilled liners, screens and gravel packs. Near-well bore cleaning by acoustic stimulation has shown great promise in laboratory experiments, and the industry has developed several tools using this technique for use in real-world wells.
One type of acoustic tool that has been described employs an oval configuration. In an oval configuration, the tool has a cylindrical housing with a set of piezoelectric drivers mounted a various locations around the side walls of the housing. Separate control signals are used to activate the individual piezoelectric drivers according to a desired mode of operation. According to one exemplary oval mode configuration, four divers are position equidistant around the circumference of the housing. A four divers make up two sets with two drivers in each set. In each set of drivers, the drivers are positioned exactly opposite from each other on the housing. During operation, a first set of drivers is activated to pull outwardly on the sidewall of the housing, while the second set of drivers is activated to push inwardly on the housing. According to separate control signals, the drivers are then activated to push/pull in the opposite directions. In particular, the first set of drivers is activated to push inwardly on the sidewall of the housing, while the second set of drivers is activated to pull outwardly on the housing. As the piezoelectric drivers vibrate, the housing flexes between an oval having its major axis along the first set of drivers and an oval having its major axis along the second set of drivers. The radiated energy is strongest at the antinodes (the wall locations intersected by the diameters having maximum deflection. Midway between the antinodes on the wall are locations that remain stationary during vibration. The stationary points are nodes. Depending on the wall thickness and the material properties of the housing and the size of the piezoelectric drivers, the drivers may be activated at a frequency equal to a harmonic frequency of the housing so that the amplitude of deflection is maximized.
The lowest frequency oval mode has four antinodes and four nodes. Higher-order oval modes have even integer numbers of antinodes and nodes (six, eight, etc.). While it is theoretically possible to operate a tool with any order of oval mode resonance by driving the tool at the resonant frequency of that order of mode, practical limits on electrical impedance matching and driver placement constrain the number of useful driving frequencies.